1. Field of the Invention
The present invention relates to a method of making a dielectric isolation structure for electrically isolating circuit elements from one another in an integrated circuit.
2. Description of the Related Art
As dielectric insulating isolation techniques for electrically isolating circuit elements from one another in an integrated circuits, it has been known to provide a field oxide film on a semiconductor substrate by LOCOS techniques. A complete dielectric isolation structure which is obtained in the following manner has been also known. That is, a first semiconductor substrate is adhered to a second semiconductor substrate through an insulating layer, and trenches reaching the insulating layer are provided in the first semiconductor substrate. Thereafter, side walls of the trenches are oxidized to form an oxide film, and the trenches are filled with a dielectric material.
In such a complete dielectric isolation technique, it is necessary to round corners of the trenches. If the trenches do not have rounded portions at corners, an unwanted stress will occur at the corners of each trench in the step of forming the oxide film on the side walls thereof, resulting from a difference of thermal expansion coefficient between the insulating film on the second semiconductor substrate and the first semiconductor substrate. e.g., between silicon dioxide and silicon. As a result, a dislocation density of silicon crystal will be increased in proportion to the stress, thereby producing crystal defects at the corners of the trenches.
A conventional technique for rounding corners of a trench will be described below with reference to FIGS. 4A to 4D.
As shown in FIG. 4A, an oxide film 12, a nitride film 13 and an oxide film 14 are successively formed on a silicon semiconductor substrate 11, and then these films are patterned to obtain a mask for forming a trench. Using the mask, the semiconductor substrate 11 is etched by RIE (Reactive Ion Etching) techniques to provide a trench 15 therein.
As shown in FIG. 4B, the oxide films 12 and 14 in the mask are side-etched by a diluted HF solution by several 100 .ANG. to expose corners 16 of the semiconductor substrate 11 in the trench 15.
As shown in FIG. 4C, the corners 16 of the semiconductor substrate 11 are removed by CDE (Chemical Dry Etching) techniques, using the oxide film 12 as a mask.
Finally, as shown in FIG. 4D, after the oxide film 12, the nitride film 13 and the oxide film 14 are removed, the entire surface of the substrate including side walls of the trench 15 is oxidized to form an oxide film 17, thereby rounding the corners 16.
In the above method of rounding the corners of the trench, important parameters for controlling the conditions of rounding the corners are both the amount of side-etching of the mask, i.e., the oxide film 12, by the diluted HF solution as shown in FIG. 4B and the amount of etching of the corners 16 of the semiconductor substrate 11 by the CDE as shown in FIG. 4C. In this case, the corners are rounded under conditions wherein the amount of side-etching of the oxide film 12 of about 500.ANG. and the amount of etching is about 1,000 .ANG. by the CDE.
On the other hand, in relation to the complete dielectric isolation structure, a trench reaching the insulating film is provided in a semiconductor substrate adhered to a base substrate through an insulation film. Thereafter, when the trench side walls are oxidized to form oxidized film 17, as shown in FIG. 4D, conditions necessary to achieve rounded upper and lower corners must be achieved.
However, when a desired complete dielectric isolation structure is to be produced the conditions that the side-etching amount of the oxide film for forming the mask is about 500 .ANG. and the etching amount by the CDE is about 1,000 .ANG., the upper corners can be rounded, but it is difficult to round the lower corners. As a result, unwanted stress will be concentrated at the lower corners of the trench to increase the dislocation density of silicon crystal. Therefore crystal defects will be produced at the lower corners of the trench.
As described above, when such a conventional technique for rounding the upper corners of the trench is intended to be applied to the complete isolation structure in which portions of the trench to be rounded are present at the upper and lower corners, it will be difficult to round the lower corners, and unwanted crystal defects will be produced in the semiconductor substrate.